1 Answer
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To drive in Spain you must be aged 18 or over. To rent a vehicle you
must be 21 or over. Many companies also require you to have held your
driver’s license for a minimum of one or two years. Remember that you
will also require a credit card to rent a vehicle.

A valid driver’s license:

If you are the citizen of an EU member state, Switzerland, Norway,
Iceland or Liechtenstein: you only require your valid driver’s
license. If you are from another country you will require an
International Driver’s License.

Given that conditions may vary, we suggest that you contact the
Spanish Embassy or Consulate to verify these requirements before you
start your trip.

Note that the IDL is only a translation of a person's already-issued driver's license, and is not a driver's license in itself. The IDL is usually only required when the driver's license has non-Roman letters (as, for example, a license from Japan). The IDL must be accompanied by the driver's already-issued regular driver's license. The paragraph in @Burham's answer would be more accurate if it stated "If you are from another country, you will also require an International Driver's License."
– DavidApr 23 at 20:37